Rotary dryers and heat exchangers are used to transfer heat from a hot gas, usually the product of combustion, to the sludge or granular material which it is desired to heat or dry. Rotary drum dryers are generally of two types. Both types employ a horizontal rotating drum through which the sludge or granulated material passes. The drum is generally slightly inclined from the infeed end to the outfeed end to cause the material being dried to progress down the drum as it rotates. Often mounted within the drum are lift flights of various types designed to agitate the material passing through the drum and/or to cast the material into a falling veil of material where it can interact with gases passing through the drum. One type of drum heater utilizes indirect heating wherein a furnace or manifold for hot gases surrounds the central portion of the drum, thus heating the exterior of the drum which in turn heats the material passing through the interior of the drum by conduction and radiation.
The other type of drum dryer employs direct heating wherein a burner or furnace at one end of the drum introduces hot combustion gases into the interior of the drum. The hot combustion gases directly transfer heat to and dry moisture from the sludge or granular material progressing through the drum. The directly fired drum heaters are divided into those in which the combustion gases flow in the same direction as the granular material through the drum, and those in which the combustion gases counterflow the granular material being dried or heated. The counterflow type dryers have the advantage of greater thermal efficiency. Counterflow dryers, because they bring the driest, warmest material directly into contact with the hot drying gases entering the dryer, have a tendency to overheat at least a portion of the material being dried. Parallel flow dryers, on the other hand, are less efficient but, because the hot drying gases first meet the cool, damp granular material being dried, are less subject to overheating the material.
Some materials such as rock aggregate are not adversely affected if a portion of the material is heated significantly above the average desired temperature of the particulate being heated. On the other hand, in the treatment of other materials which may be desirable to heat or dry in a rotating drum dryer such as recycled asphalt, wood chips, agricultural products and pulp mill sludge, overheating even a small portion of the material can result in charring which may adversely affect the product being heated and will often release large quantities of particulate matter and volatiles in the form of smoke. Where air quality is a concern, the cost of removing the particulate and particularly the volatile will often indicate the use of the less efficient parallel flow rotary drum dryer, which to a lesser extent is still prone to generating smoke from materials with low char temperatures.
Indirect rotary drum heaters can be used to supply a more uniform heating to the contents of a drum dryer, and may also be used where it is desirable not to contact the material being processed with combustion gases so as to avoid contamination of the processed material. Indirect rotary drum dryers, however, are limited in efficiency. It is more difficult to make the gases in the oven flow in counterflow to the material inside the drum, as the gases in the oven or manifold surrounding the rotating drum have a tendency to rise to the top and generally to be churned by the moving drum surface, thus preventing the heating gases from moving in uniform counterflow to the material inside the drum. Further, because the heating takes place through the drum, a significant temperature differential must be maintained between the external gases and the material moving through the drum to cause significant amounts of heat to flow from the gases into the materials, thus the exhaust gases of an indirectly heated drum necessarily carry with them considerable wasted energy.
One known type of drum dryer employed in drying pellets of carbon black utilizes an indirectly heated drum dryer with a jacket surrounding the center of the drum to which a heating medium such as combustion gases are supplied. This known drum heater also employs a small stream of hot gases moving in counterflow through the drum for the purposes of purging liquid vapor from the drum. Other known rotary drum dryers employ both indirect heating by means of a jacket or oven surrounding the drum and direct firing by means of a burner at one end of the drum. Still other known types of rotary drum dryers employ indirect heating by means of an oven surrounding the drum and by means of perforations in the drum which allow the entry of combustion gases from the oven into the drum along its length.
Dryers by their nature evaporate large quantities of water, an energy intensive process which absorbs approximately 1,000 btu's per pound of water evaporated. In many situations employing a rotary drum for heating or drying, the majority of the energy utilized goes into the evaporation of water. Known rotary drum dryers do not have any means for recovering any portion of the latent heat contained in the driven-off water vapors.
Further, known rotary drum dryers and heaters often require expensive pollution control equipment such as baghouses to control the emission of dust and smoke from the dryer.
What is needed is a rotary drum heater/dryer with large capability, high efficiency, uniform heating and minimized emissions.